1. Field of the Invention
The present invention relates to an automatic power control apparatus for stabilizing a level of output power in a radio communication system such as a cellular radio telephone system.
2. Description of the Related Art
In a radio communication system such as a cellular radio telephone system, generally, automatic power control (referred to as APC hereinafter) is performed to stabilize a level of output power while the system is operating.
FIG. 4 is a block diagram showing the arrangement of a conventional APC apparatus. In FIG. 4, reference numeral 1 denotes a variable amplifier for amplifying a transmit signal output from a transmitter (not shown). The transmit signal is supplied to, for example, an antenna (not shown) via the variable amplifier 1. The transmit signal is also supplied from the variable amplifier 1 to a level detector 2. The level detector 2 outputs a detection signal having a voltage corresponding to a power level of the transmit signal. The detection signal is then input to a comparator circuit 3 comprising a differential amplifier 31, a variable resistor VR, and resistors R1, R2 and R3. A reference voltage VB is also applied to the comparator circuit 3 from a variable power source 4. The comparator circuit 3 compares the voltage of the detection signal output from the level detector 2 with the reference voltage VB and then supplies a difference between the voltages to a control voltage delivering circuit 5 as a comparison output voltage.
The control voltage delivering circuit 5, which is constituted of a Darlington circuit, amplifies the comparison output voltage output from the comparator circuit 3 and supplies the amplified voltage to the variable amplifier 1 as a control voltage of the amplifier 1. The variable amplifier 1 amplifies the transmit signal at a gain corresponding to the control voltage supplied from the control voltage delivering circuit 5.
Consequently, the transmit signal output from the variable amplifier 1 is always stabilized so as to have a fixed power level which depends upon the reference voltage VB preset in the variable power source 4.
Digital modulation for modulating an analog carrier by a digital signal has recently been popularized in a radio communication system. For example, quadrative amplitude modulation (referred to as QAM hereinafter) system is generally known as the digital modulation. The use of such digital modulation enables so-called time-sharing transmission. For effective use of channels, a time division multiplexing access (referred to as TDMA hereinafter) system has been adopted.
A transmitter of a radio communication system to which the QAM and TDMA are applied, will be outlined. In the TDMA, the transmitter performs a transmission operation only during the time slots assigned to the transmitter itself. When the transmission operation is performed, a predetermined period t0, during which a non-modulated wave is output, is set in the initial part of each of the time slots as shown in FIG. 5(b), and the APC apparatus automatically controls power so that the non-modulated wave is adjusted to have a predetermined level P during the period t0.
In the QAM, generally, an instantaneous level of a modulated wave varies with digital data to be transmitted. More specifically, when the QAM is performed using a digital pulse, the digital pulse is caused to pass through a low-pass filter to restrict the bandwidth of the digital pulse. Since the digital pulse is expanded with respect to time by the restriction of bandwidth, the waveform of the pulse is influenced by its preceding and succeeding pulses. This means that the level of a non-modulated wave varies with a modulation signal of a modulated wave subsequent to the non-modulated wave. Since the ratio of an average level of a signal transmitted during the time slot to that of a signal transmitted during the non-modulated wave output period changes with each of the time slots, an error .DELTA.P occurs in the average level between the time slots as shown in FIG. 5(b). FIG. 5(a) is a view showing time slots in which TDMA signals are generated, and the hatched portions in FIG. 5(a) show time slots assigned to the transmitter. FIG. 5(b) is a view showing the waveforms of transmit signals output from the variable amplifier 1.
As described above, when the conventional APC apparatus is applied to a radio communication system wherein linear modulation such as the QAM is used as a modulation system and the TDMA is used as a multiplexing access system, an error occurs in average power every time slot, and automatic power control cannot be correctly performed.